Process for making a stress-free plastic article

ABSTRACT

Disclosed is a method and apparatus for making stress free plastic articles from plastic particles such as powder utilizing heat and pressure in which a quantity of plastic particles is supplied to a first member, a flexible diaphragm is disposed in opposed relationship to said first member with said plastic particles therebetween, a microporous air release means is positioned between said plastic particles and said flexible diaphragm, a fluid-like pressure is applied to said diaphragm to cause said diaphragm to apply an even fluid-like pressure through said air release means to said plastic particles, heat is applied to said plastic particles while under said fluid-like pressure and any entrapped gases between the particles is vented by means of the air release means. The pressure and heat are controlled to effect a consolidation of the plastic particles into the article. A second diaphragm may be used on the opposite side of said first diaphragm and either one or both of said diaphragms may be a sealed envelope to which fluid pressure is applied. The plastic article produced by the method is substantially free of all stresses.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. Pat. No. 4,243,368, issued Jan.6, 1981, which No. 929,304, filed July 31, 1978 which application is acontinuation-in-part of Ser. No. 647,832, filed Jan. 9, 1976, and isalso a continuation-in-part of Ser. No. 817,048, filed July 19, 1977.The history of these two parent applications is as follows:

i. Ser. No. 647,832 was filed Jan. 9, 1976 (abandoned) as a continuationof Ser. No. 424,654, filed Dec. 14, 1973 (abandoned) as a continuationof Ser. No. 141,495, filed May 10, 1971 (abandoned); and

ii. Ser. No. 817,048 was filed July 19, 1977 (now U.S. Pat. No.4,104,101) as a continuation of Ser. No. 279,964, filed Aug. 11, 1972(abandoned) as a continuation-in-part of Ser. No. 141,495, filed May 10,1971 ( abandoned);

U.S. Serial No. 141,495, filed May 10, 1971 (from which both saidapplications Ser. Nos. 647,832 and 817,048 are derived) was filed as adivisional of Ser. No. 805,093, filed Feb. 14, 1969 (abandoned) as acontinuation of Ser. No. 536,140 filed March 21, 1966 (abandoned) as acontinuation-in-part of Serial No. 473,342, filed July 20, 1965 (nowU.S. Pat. No. 3,383,265). The references cited in above U.S. Pat. No.4,243,368 are noted for the record.

FIELD OF THE INVENTION

This invention belongs to the field of forming plastic articlesincluding sheets from plastic particles and to welding the same underheat and pressure also by utilizing plastic powders. Previous knownmethods of forming plastic articles including sheets from plasticparticles have experienced a variety of difficulties. Many plastics aredifficult or impossible to mold into articles or to form into sheets,particularly, thin sheets called films. Also, it is difficult to weldmany plastic sheets by conventional electronic heating or other"thermowelding" techniques without having the joint exhibit a thicknessgreater than the thickness of the sheet. Further, such techniques forwelding have generally been applicable only to certain relatively thinmaterials such as films. For example, the homopolymerchlorotrifluoroethylene may be thermowelded only in very thinthicknesses. In such conventional welding techniques the resultingjoints frequently exhibit varying strengths and degradation of theplastic along the weld due to excessive or unequal heating, or unequalpressure. Still further, such conventional welding techniques frequentlyresult in holes through the material at or adjacent the weld eitherduring formation or subsequently during use which phenomenon issometimes referred to as "holing through". When the sheet material to bewelded has variable thicknesses these difficulties in conventionalwelding techniques are compounded even when the thickness variation isslight. Nor do the conventional welding systems produce a weld over theentire width of the overlapped edges. As a result, the unwelded portionsof the edges are subject to movement in use by wind or other aircurrents which deteriorate the weld, often to the point where the weldseparates. Previous efforts to overcome these problems with conventionalwelding by using wider welded areas (more overlap) greatly reduces thespeed of joining which is detrimental commercially.

Much of what has been said above with respect to welding also applies tothe forming of shaped articles including sheets from plastic particles.Still further, when forming plastic articles or sheets from plasticparticles by the use of conventional methods the articles or sheetsproduced have stresses or strains built into them by the method offormation except in those instances where the plastic being used is amonomer or a liquid-like material or are formed by solvent or castingmethods such as those methods using organosols or plastisols for theproduction of the article or the sheet. Solvent methods of formation areexpensive, generally slow, and are not applicable to a great manyplastic resins. Polymers having millions molecular weight, for example,have proven very difficult to form into articles or sheets by manyconventional techniques and, indeed, it is practically impossible toproduce large thin sheets from such millions molecular weight polymersdirectly from the plastic particles. While certain plastics have beenformed into articles, particularly sheets, that are nearly stress orstrain free by the solvent or liquid phase methods mentioned above, suchprocesses have not been widely used with solid polymers except withpolymers such as polyvinyl chloride. Still further, such stress orstrain free products (hereinafter referred to as "stress free" products)have not heretofore been produced from polymers having millionsmolecular weight. In an effort to produce stress free plastic articlesincluding sheets, attempts have heretofore been made to release thestresses built into the product during its formation by certainexpensive after-treatments such as heating the product to a temperaturesuitable to relieve the stresses therein. Generally this involvesheating the product to near and sometimes into the melt phase forextended periods. Such treatments (especially if the product is notrestrained during treatment) can induce dimensional changes in theproduct which render it useless. The present invention eliminates stressas a problem. Prior to the present invention the best quality of sheet(including film) plastics produced from pelletized or ground plasticpowder has been made by the compression method; however, due to itslarge capital investment and costly operation such sheets produced bysuch compression method have priced themselves out of the market placeexcept for very special applications. In the compression method thepressures required may reach many thousands of pounds per square inchwith the recommended pressures for polystyrene homopolymer sheets, forexample, being from 1,000 to 10,000 p.s.i. (70.3-703 kg/cm²) and thosefor acrylics being on the order of 2,000-5,000 p.s.i. (140-352 kg/cm²).

BACKGROUND ART

It is known from U.S. Pat. No. 3,383,265 that two plastic sheets can bewelded together by placing them in a chamber having at least oneflexible wall and heating with infrared or other heat while the flexiblewall is pressed against the joint until it melts into the weld. Suchprocess and method are, however, not disclosed as being useful forforming products from particulate material, much less stress freeproducts.

DISCLOSURE OF THE INVENTION

The present invention proposes to overcome the deficiencies of the priorart by producing plastic articles which are stress free directly fromplastic particles by supporting said particles upon a suitable supportsuch as a mold or platen, heating the plastic particles while applyingto said plastic particles fluid pressure including applying said fluidpressure substantially uniformly to said supported plastic particleswhile the same are heated to a temperature sufficient to cause saidparticles to melt and form said article, and venting any air of gassescontained in the plastic powders during said heating and pressing. Theapparatus for performing the method utilizes a flexible member which maybe a diaphragm alone or a diaphragm that is one face of a closedenvelope. The apparatus also includes an air release means which is,preferably, in two parts although a single element air release means isalso contemplated. When the air release means is of two parts, one is awire cloth or screen and the other is a microporous parting sheet. Themicroporous parting sheet may advantageously be comprised of afiberglass fabric coated with polytetrafluroethylene, thus providing asheet with a myriad of minute openings therethrough.

In using the apparatus of this invention in carrying out the method toproduce stress free articles such as sheets, the pressures required areoften as low as 50 p.s.i. (3.52 kg/cm²) and less including use of onlythat force generated by the differential between atmospheric pressureand a readily obtainable partial vacuum.

The articles including sheets produced by this invention are stress freeand degradation is greatly reduced or eliminated. The articles may bemade of such diverse plastics as polyethylene, polyvinyl chloride,polystyrene, polypropylene, the acrylics, and others, including,particularly, polymers of millions molecular weight such as millionsmolecular weight polyethylene, which are otherwise very difficult toprocess. Dissimilar plastics even such widely dissimilar materials suchas the polystyrenes and polyvinyl chlorides may be superimposed in oneor more layers and produced as one solid sheet.

Still further, the product produced by this invention exhibits thecharacteristics on its surface of the mold or platen support. In onespecific example the product is produced having a mirror surface ofextremely high quality which exhibits high smoothness or polishcomparable to that of polished glass by having been formed on a platenhaving a surface with an extreme polish. Such a surface may be glass orhighly polished metal and may, if desired, have patterns engraved orotherwise produced therein for reproduction in the final plasticarticle.

In carrying out the method to produce such a sheet suitable for use as amirror the plastic material in comminuted state is disposed on asuitable mold or platen in a layer of desired thickness. Over this layerof comminuted plastic particles or powder is placed the above-mentionedmicroporous parting sheet and over this is placed the wire screen. Theplastic is then heated to at least its fusion or melt temperature andwhile so heated there is applied across the gas release means (comprisedof the microporous parting sheet and the wire screen) a fluid-likepressure sufficient to cause the plastic material to fuse to a coherentsheet. The fluid pressure applied to this sandwich may be exertedthrough a flexible diaphragm or by means of two separate diaphragms oneof which is placed beneath the platen and the other of which is placedabove the wire screen. Fluid pressure is then applied to the diaphragmto cause the same to press evenly in a fluid like manner over the entiresurface of the layer of plastic particles.

The diaphragm may be either a single flexible member placed against oneor both sides of the sandwich (comprising the mixture, plasticparticles, microporous parting sheet, and wire screen) or the diaphragmmay be one face of a sealed envelope in which case either a singleenvelope pressing against the air release means or a pair of opposedsealed envelopes (with the sandwich therebetween) may be utilized.

The pressure may be either liquid such as water or hydraulic fluid underpressure or gas such as air or an inert gas under pressure. In eitherevent, the fluid pressure may be applied to the outer surfaces of thediaphragm or diaphragms which transmits the same to the plasticparticles. Alternatively, and particularly when two diaphragms are usedwhich are not the faces of sealed envelopes, the periphery of thediaphragms may be sealed together and the air between the two evacuatedwhereby the pressure is ambient pressure and the amount thereof isdetermined by the differential between ambient pressure and the degreeof vacuum applied between the peripherally sealed diaphragms. When oneor two envelopes are used, the fluid pressure is applied by placing thefluid within the envelope under pressure which pressure the flexiblediaphragm face of the envelope transmits fluidly to the plasticparticles.

During the step of applying fluid pressure (by any of the describedmechanisms or methods) to the heated plastic material, all areas of theplastics material are contacted with a pressure that is even andsubstantially uniform throughout. Any gas (including air or moisture)present in the plastics material is exhausted under the heat andpressure and the same is vented by means of the air release meanscomprised of the wire cloth or screen and the microporous parting sheet.It should also be noted that a porous or multiperforated thin flexiblemetal sheet may be used in place of the wire screen. Alternatively, apolytetrafluoroethylene (PTFE) coated wire screen or minutely perforatedor porous flexible metal sheets coated with PTFE may be used to replaceboth the wire screen and the microporous parting sheet.

As the plastics material melts and consolidates, the significantmovement is that towards solidification thus avoiding the gross lateralflow that in other processes causes the introduction of various stressesinto the product upon cooling. If the sheet is produced on a glass platemaster the fluid pressures applied are so even that the product isproduced without any damage or breakage occurring to the glass master.The glass master is preferably tempered or heat resistant glass.

When producing a mirror, as one specific embodiment, there is thenapplied to that face of the finished sheet which exhibits the polish ofthe master (i.e. that face which was against the master) a layer ofreflective metal. The reflective metal may be sprayed silver and inorder to protect the same there is applied thereover a transparentabrasion resistant coating. This abrasion resistent coating mayadvantageously be a solution of glass resin polymer applied evenly overthe metal layer which resin is then cured by means of radiant heat. Inone specific example the plastic sheet may be a polystyrene homopolymerof approximately .09" (2.29 mm) in thickness with a layer of silversprayed onto its polished surface and the silver layer coated with afilm of 40% glass resin and 5% catalyst in solution in butanol.

When welding sheets of plastics together previous disadvantagesmentioned above including "holing through" are eliminated whileproducing a weld with little or no thickness variation being apparent inthe final product not even in the weld and also having a mirror or othersmooth surface as desired. Further, the invention permits the welding ofdissimilar plastics, even those as diverse as polyethylene and polyvinylchloride.

When the invention is used to join two sheets or pieces of plasticmaterial, they may be placed in juxtaposed edge to edge relationship orin overlapped relationship with powdered plastic between the meeting oroverlapped edges. A fluid-like pressure is then applied to the jointwhile heating the same into or close to the melting temperature. As inthe case of producing an article, the fluid pressure is applied byeither one diaphragm or by two opposed diaphragms either or both ofwhich may be one face of a sealed envelope. Under the applied heat andpressure the weld is formed and the particles lose their discreetidentities and assume the ultimate desired shape.

Where the materials to be joined are different plastic polymers the useof correspondingly different plastic powders has been found particularlyadvantageous. For example, different plastic powders have been used inwhich the plastic of one powder corresponds to the plastic of one ofsaid polymers and the plastic of the other powder corresponds to theplastic of the other of said polymers. At the joint or junction betweenthe two materials the plastic powders may advantageously be arranged ina graded manner. That is to say that the powder adjacent the first ofsaid materials is comprised of the same plastic as the first of saidmaterials while the plastic powder adjacent the second of said materialsis the same polymer as the second plastic polymer. Between theseextremes the concentration of one of said powders increases and theother decreases in a gradual manner until midway between the twomaterials to be joined the powder comprises a mixture of substantiallyequal amounts of the two different polymers. The entire assembly is thensubjected to a uniform fluid pressure with applied heat to affect awelding of the joint and a melting and fusion of the powders with eachother and with the adjacent (juxtaposed or overlapped) edges of thematerials being joined. Alternatively, in many applications the powdersmay be supplied at the joint without such gradation toward the center.

After the application of the plastic powders to one of the platens theplatens are moved toward each other until a light pressure is applied tothe layer of plastic particles. Thereafter the pressure is applied tothe diaphragm or the envelope as described above. The diaphragm contactsthe plastic powder with an even fluid pressure while heat is applied.The fluid pressure is substantially uniform over all of the plasticparticles while the applied heat is sufficient to melt and consolidatethe particles of the powder. It will be seen that the same basic methodis used whether an article, a sheet, a weld or other joint is beingproduced.

The time required to form the article including a sheet or to makesuitable welds will in part depend upon the particular plastic orplastics being worked, the thickness of the final product and likeconsiderations. After applying heat and pressure as described above fora sufficient length of time to produce the article the pressure isreleased from the diaphragm (or envelope) and the platens are separatedpermitting the removal of the product.

There is far less or practically no degradation of the plastic whetherin forming an article or making a weld since the diaphragm or diaphragmseffectively exclude virtually all air from contact with the plastic. Theair release means such as the PTFE coated glass fiber fabric having manyminute pinholes and the wire cloth or screen described above also helpsthe escape of air so that the plastic is formed or welded in the absenceof air with the result that degradation is greatly reduced oreliminated.

The apparatus used in carrying out the method comprises a frame havingmounted thereon upper and lower platens at least one of which is coveredwith a flexible impermeable diaphragm or envelope. The diaphragm orenvelope may be of any suitable material which will withstand thepressure and heat of operation including suitably selected metal sheets,plastic sheets, plastic and fabric combinations or the like. Metalalloys having extremely low coefficients of expansion are particularlyuseful for the diaphragm or the diaphragm wall of the envelope. When thediaphragm or diaphragm face of the envelope is of metal thicknesses fromabout 0.002" (0.05 mm) to 0.0075" (0.19 mm) have been found adequate.However, thicker diaphragms may be used as well with up to about 0.06"(1.5 mm) being contemplated. Thicknesses of 0.020" (0.51 mm) to 0.035"(0.89 mm) are generally sufficient.

The heat may be supplied to one or both platens in a number of ways.Common heat bars or heat elements of the electrical resistance type aresatisfactory in many applications although a more versatile means ofsupplying heat is by using infrared radiation in wave lengths from 7,500A to 6 microns or more. Since such infrared radiation lends itselfreadily to concentration, absorption, transmission, and refraction overthe areas to be heated by use of known optical and physical means suchinfrared radiation heating is more versatile than ordinary electricalresistance heating elements. In either event with either system quiteaccurate temperature control is possible from less than about 200° F.(94° C.) to an excess of about 2,000° F. (1093° C.) which lattertemperature far exceeds any normal temperature requirement when workingwith plastics.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention will be fully understood by those skilled in the art fromthe following description and drawings in which:

FIG. 1 illustrates schematically the production of a wide sheet by useof plastic powders;

FIG. 2 illustrates a mixture of different plastic powders gradedprogressively when joining two dissimilar plastic sheets;

FIG. 3 is a schematic vertical section of another embodiment of theapparatus of the invention;

FIG. 4 is a schematic vertical section of yet another embodiment of theapparatus of the invention; and

FIG. 5 is a schematic side elevation illustrating the curing of anabrasion resistant coating applied to the silvered face of a plasticsheet when producing a mirror in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, there is shown an apparatus 100 comprising a lower platen 80equipped with heating elements 81. The heating elements 81 may beelectrical resistance heaters or, alternatively, may be tubes throughwhich a heating fluid is pumped. A second platen 88 is positioned inopposed relationship to the platen 80. The platen 88 also incorporatesheating elements 89 either by way of resistance heaters or tubes throughwhich tube heating fluid is pumped. Any of a number of suitable heatsources may be used in place of or in addition to the heating cores81,89. For example, infrared radiation from any suitable source such aselectrical bars, quartz tube heaters or the like are particularlyversatile because of the ease of controlling, concentrating,transmitting and absorbing the same on the material to be heated by useof known optical and physical techniques. However, any source of heatcan be adapted for use in this invention.

The platens 80 and 88 are mounted for relative movement toward and fromone another by movement of one or both thereof. Secured to one of theplatens 80,88, preferably the upper platen 88 as shown, is a flexibleenvelope 86 containing a fluid 87. The envelope 86 includes a front faceor diaphragm 90. The diaphragm 90 and the entire envelope 86 may be ofany suitable material which will withstand the pressure and heat ofoperation including suitably selected metal sheets, plastic sheets,plastic and fabric combinations or the like. In some instances, metalalloys having extremely low coefficients of expansion are particularlyuseful for the diaphragm and for the entire envelope. However, in mostapplications it has been found that high coeffcient of expansionmaterials, particularly copper and some stainless steels are verysuitable. When a metal is used it has been found that thicknesses fromabout 0.020" (0.51 mm) to 0.035" (0.89 mm) are adequate. However,thicknesses of up to about 0.06" (1.5 mm) are also useful. The interiorof the envelope containing the fluid 87 communicates through a suitableconduit 91 with a source of fluid under pressure (not shown). The fluidmay be a liquid such as water, hydraulic fluid, heat transfer fluid orthe like or, alternatively, the fluid may be gaseous in which event air,or an inert gas such as nitrogen are suitable. The pressure source isselected depending upon the type of fluid (liquid or gaseous) which isto be utilized. A polished plate 82 is placed upon the platen 80 for theformation of a sheet of plastic. It will be appreciated that withsuitable mold release agents the polished plate 82 may be dispensed withand the sheet formed directly upon the platen 80; however, the polishedplate 82 is preferred. Members 102 and 104 on the plate 82 confine theplastic particles 83 which preferably comprise a finely ground powder.As will be apparent to those skilled in the art, in a batch typeoperation where a single sheet is to be formed the members 102 and 104extend around the periphery of the platen 80 and the plate 82 to confinethe plastic particles 83 on all sides. However, when a continuous orsubstantially continuous sheet is to be formed the platens 80 and 88 canbe elongated and comprise the adjacent converging runs of twoconveyor-like structures so that the platens 80 and 88 not only movetoward and from each other but also move longitudinally along with eachother at the same rate. In the latter event, the members 102 and 104 arepositioned only at the lateral side edges of the platen 80 as shown.

The apparatus also includes a breathable microporous sheet 84 which isplaced immediately above the plastic particles 83. The sheet 84comprises a fabric material made of fiberglass and coated with PTFE insuch a manner that the sheet has a myriad of minute pin holessubstantially invisible to the naked eye extending therethroughthroughout substantially the entire extent of the sheet. Positioned overthe microporous parting sheet 84 is a fine woven wire cloth or screen85. Alternatively, the element 85 may be a thin, flexible, porous orminutely perforated metal sheet. As will be apparent in the followingdescription, the microporous parting sheet 84 and the wire screen 85together comprise an air release means to release entrapped air from thelayer of plastic particles 83 during formation of the sheet. Themicroporous parting sheet 84 and the wire screen 85 may be single sheetsof material placed over the particle layer 83 in a batch operation oreach may be a continuous track of material in the event the sheet is tobe formed substantially continuously.

While the above description refers to a two part air release means, aone part air release means as above described is also contemplated. Insuch case the elements 84 and 85 are replaced by a single PTFE coatedsheet comprising a porous metal sheet or wire screen.

In operation, a layer of plastic particles 83 is spread substantiallyevenly in the area defined by the members 102 and 104 which layer isspread to a thickness substantially equal to or slightly greater thanthat of the members 102 and 104. The heating elements 81 or 89 or bothare then energized to heat the platens 80 or 88 or both, and in the caseof the platen 88 in order to heat the fluid 87 and the diaphragm 90 aswell. While either platen 80 or 88 may be heated as described or bothmay be heated, it is sometimes sufficient that the lower platen 80 beheated whereby the heat is transferred through the polished plate 82 tothe plastic layer 83. The platens 80 and 88 are then brought toward eachother with the microporous parting sheet 84 and the wire screen 85positioned therebetween. It will be appreciated that the layer ofparticles 83 together with the polished sheet 82, microporous partingsheet 84, and wire screen 85 comprise a "sandwich" located between thetwo platens 80 and 88.

When the platens 80 and 88 have been brought together sufficiently sothat the diaphragm 90 is applying a firm pressure on the layer 83 ofplastic particles through the wire screen 85 and microporous partingsheet 84, then pressure is supplied through the conduit 91 to theenvelope 86 and thus to the diaphragm 90. Under the heat supplied by theplaten 80 (or the platen 88 or both) and the pressure applied by thediaphragm 90 under pressure from the fluid 87, the plastic particlesmelt and consolidate into the plastic sheet. There is no lateral flow ofplastic, only movement together to consolidation thus eliminating flowlines that produce stress. During this operation the diaphragm 90insures that all areas of the layer 83 of particulate plastic materialare under uniform pressure. After a suitable time, the platen 80 and 88are separated, and the sheet produced from the layer of particles 83 isremoved from the apparatus.

The particle size is preferably below about 40 mesh, although,generally, the finer the powder used the better the results obtained,but, in any event, the powder should be free flowing so that there is aneasy leveling of the plastic particle layer when the particles areapplied initially. The pressures used will generally be below about 50p.s.i. (3.52 kg/cm²), with 40 p.s.i. (2.81 kg/cm²) being typical thoughthis will vary depending upon the material. The temperature best used isusually above the melt and will depend upon the particular plasticparticles being utilized with 350° F. (177° C.) being useful forpolyvinyl chloride (PVC) as polymerized (without additives) andcommercially available.

This type of process produces a sheet that is vastly superior toprevious products, which is stress free and has greatly improvedphysical and mechanical properties. Surprisingly, sheets of plastic madefrom the highest molecular weights such as millions molecular weightpolyethylene (MMwPE) are thermoformable when made by this process.Additionally, there is little or even no degradation even with materialsin which high degradation is normally considered unavoidable. Forexample, MMwPE produces a stress free sheet that is completelyundegraded and can be produced in any thickness ranging from very thinfilms to heavy weights. Among other attributes it is this freedom fromstress that makes these MMwPE sheets thermoformable whereas such sheetsproduced by other methods (if they can be produced at all) are verydifficult or impossible to thermoform.

Pure polyvinylchloride (PVC) sheet or film produced from polymerizedplastic particles exhibits previously unobtainable properties. Amongthese are great impact strengths at far below freezing comparable tothat at room temperature and the lack of the high degradation in a sheetof pure PVC which it is otherwise not possible to produce except byadding expensive additives. As another example, PTFE articles producedby this invention exhibit a much superior dielectric strength. By thisprocess and equipment practically the whole range of thermoplasticmaterials can be processed.

Two or more dissimilar materials can be spread in layers to produce acladding, laminating, or surfacing thus allowing different properties tobe combined at low cost or, alternatively, incorporating propertiesotherwise unattainable in one material. As one example, it is possibleto produce a laminate in one step. The plastic particle layer 83 mayinclude at the upper surface thereof a second layer 83' of a differentplastic polymer in order that the completed product comprise a plasticsheet having one plastic on one side and a different plastic on theother side. Still further, a wood, metal, or other like material may beplaced directly upon the polished plate 82 or the platen 80 and a layerof plastic particles 83 distributed thereover and then formed by thisprocess such that the plastic particles are laminated directly to thewood, metal or other layer. Still further, one or more differentpolymeric materials can be spread in layers of the same or differentthickness and simultaneously formed into film or sheet in one step. Forexample, normal molecular weight polyethylene may be spread in a layer83' over a layer 83 of MMwPE placed on the polished sheet 82 in order toprovide a cheap substrate (the upper layer 83') for facings (the layer83) having low coefficients of friction. Various mixes of powders havebeen mixed with the plastic particles in layer 83 which have includedgraphite or molybdenum disulfide also for low coefficients of frictionmaterials.

Still further, this invention permits the production of much widersheets than those which are currently possible and at a lower cost,while also eliminating great problems in maintaining thicknesstolerances. And, as indicated, products with unusual properties may beproduced. For example, very large building panels of pure PVC powder maybe used in layer 83' to provide a low cost high strength backing withsuperior impact strength onto which a weather-side surface may beproduced simultaneously in one step by providing the layer 83 with asuitable powdered plastic of an acrylic or the like which isultra-violet light absorptive. This provides unparalleled resistance tothe elements on the exposed surface. The outer surface can also be madehighly chemically resistant by using fluorinated materials in the layer83.

Due to the broad applicability of this invention plastic articles havinga wide variety of unusual characteristics may be produced. Indeed, it ispossible to "engineer" many of the properties of the final product notpreviously possible. For example, this invention includes the ability tolower the heat requirement when melting crystalline materials such aspolyethylene by as much as 20-25%. This results in a fast heating of thesheet when thermoforming with attendant savings in energy costs. Stillfurther, the same sheet may be pellitized or ground for use as rawmaterial for extrusion and injection processes and this raw materialwill require less total heat input to melt the plastic, thus making fora more economical operation.

It has even been found that some crystalline materials such aspolyethylene, chlorotrifluoroethylene, polypropylene and the like may beprocessed in the sheet at below the melt temperature. It is onlynecessary that the temperatures in the large crystal growth range beheld long enough in the compacted fine particles so that the crystalswill grow across boundaries of the sufficiently small particles and forminto a solid mass.

During the formation of the sheet from the plastic material 83 or 83plus 83' any gases including air that surround the particles escapethrough the microporous parting sheet 84 and the wire screen 85 to theatmosphere. This is true also of any moisture that may be present whichis vaporized and escapes in the same manner. Accordingly, as the sheetis being formed, it is formed substantially in the absence of air,moisture and oxygen and degradation is greatly lessened or avoided.Still further, there is no stress in the product since there is verylittle, if any, actual movement beyond that necessary to move from theparticulate form into the solidified mass of the sheet, as mentionedabove.

In producing articles with the equipment and method of this invention,efficient results are often obtained from heating the bottom platen 80only and using air pressure in the flexible envelope 86. As mentioned,however, the upper platen 88 may be heated by a suitable means. Stillanother alternative is to provide steam or hot liquids as thepressurizing fluid 87 in order to heat the diaphragm 90.

In addition to the formation of articles, in particular sheets ofplastic from plastic particles, the invention is also applicable to thewelding of previously formed plastic sheets whether of the same or ofdifferent materials. For example, not only is it possible by thisinvention to make articles at far lower pressures (at 50 p.s.i. (3.52kg/cm²) or less) than materials such as MMwPE have heretofore required,but by an adaptation of this process plastics which it has beendifficult or impossible to weld may be welded with an excellenceheretofore unknown.

FIG. 2 shows the manner of welding a butt joint between two sheets ofpreviously formed plastic sheet material 200 and 202. As shown in FIG. 2only the lower platen 80 and the polished metal plate 82 are shown forsimplicity. It will be appreciated that the upper platen 88 with itsassociated envelope 86 and diaphragm 90 together with the microporousparting sheet 84 and the wire screen 85 are also utilized in FIG. 2. Thetwo sheets 200, 202 of plastic material to be joined are positioned onthe polished plate 82 with their edges in spaced relationship as shown.In the space between their edges there is provided the plasticparticulate material 201 to a depth to provide sufficient material forthe proper thickness in the finished product to avoid either too thin ortoo thick a quantity of material at the finished joint. The particulatematerial 201 is preferably of the same polymer as the sheets 200 and202. After supplying the material 201 to the space between the adjacentedges of the sheets 200, 202, the platens 80 and 88 are brought togetheras described above to provide a firm pressure on the particles 201. Atleast one of the platens 80 or 88 (preferably the lower platen 80) orboth are heated and are maintained at a suitable temperature generallyat or above the melt temperature of the plastic particles 201. After thediaphragm 90 has been brought to bear through the wire screen 85 and themicroporous parting sheet 84 on the layer of plastic particles 201,pressure is supplied through conduit 91 to the fluid 87 and thediaphragm 90 to provide the requisite pressure for formation. After theplastic particles 201 have melted and formed the joint between thesheets 200 and 202, the pressure in envelope 86 is released and then theplatens 80 and 88 are separated, and the polished plate 82 with theproduct thereon is removed from the apparatus. If no plate 82 is usedthen the product formed on platen 80 is usually cooled before it can beremoved. For certain plastics or for reinforced articles cooling is notnecessary. It will be appreciated that this apparatus and process isessentially the same as that described above with respect to FIG. 1 forthe formation of a sheet excepting only that the elements 102 and 104are not required and are instead replaced by the sheets 200, 202 ofplastic material.

When the sheets 200 and 202 are of different plastic polymers it ispreferred that the plastic particulate material 201 be composed ofparticles of the same two plastic polymers as the sheets 200 and 202. Inmany instances it suffices to spread the two different plastic polymerparticles and have them meet directly in the joint 201; however, it issometimes preferred to use a gradual gradation from the plastic on oneside to that of the other. For example, assuming that sheet 200 is madeof a first plastic polymer designated A' and the second sheet 202 ismade of a different plastic polymer designated B', then a track ofpowders would be placed adjacent the sheet 200 which is entirely apowder of the A' polymer. Similarly a track of powder would be placedadjacent to the sheet 202 which is entirely of the B' polymer. Adjacentto the track of A' polymer would be placed a mixture of plasticparticles comprised of, say, two-thirds of the A' polymer plus one-thirdof the B' polymer and adjacent to the track of B' polymer powder wouldbe placed a track of powder comprising a mixture of, say, two-thirds ofthe B' polymer plus one-third of the A' polymer. In the middle a mixtureof one-half of the A' polymer and one-half of the B' polymer isprovided. After supplying these tracks of plastic particles the sheets200, 202 and the powder 201 are processed as above described for thejoining of two sheets 200, 202 of the same polymer. While the ratios oftwo-thirds, one-third; one-half, one-half; and one-third, two-thirdshave been mentioned by way of illustration, it will be appreciated thatother proportions may be used and fewer or more tracks with lesser orgreater gradual increase and decrease of the respective polymers may beutilized.

While the above description of the use of the invention for weldingsheets of plastic material together has referred to the forming of butttype joints, and while a butt type joint is shown in FIG. 2, theinvention is not so limited. It has been found equally as useful informing overlapped joints in which the plastic particles are placed inone or more layers between the overlapped edges of the sheets. Thelayers of plastic powders may be graded with varying proportions asabove described for the graded tracks of powders.

In the above description of welding two like or unlike pre-existingsheets of plastic together, the plastic powders used would form the weldwith the adjacent sheets using the present invention. However, itsometimes happens that a plastic powder will not form a weld with apre-existing sheet even when that sheet was produced from the sameplastic as the powder. In order to join two unlike sheets together insuch instances, the joint is formed at the same time as the sheetsthemselves using the present method and equipment.

The Examples below are illustrative of the application of the inventionto the joining of unlike sheets at the same time that one or both of thesheets are formed.

EXAMPLE I

Pure polyvinyl chloride (PVC) was joined to millions molecular weightpolyethylene (MMwPE). Pure PVC powder which was pure as polymerized (noadditives) was spread to be formed into a sheet and pure finely powderedMMwPE powder was spread to become a sheet with the powders meeting atthe middle. The two powders met directly without the use of any mixtureor gradation of the powders therebetween other than what mixing may haveresulted from the pressure subsequently applied. The materials assembledas just described were then treated as above described in an apparatussuch as illustrated in FIGS. 1 and 2 and heated to a temperature abovethe melt of either plastic, in this case, at 392° F. (200° C.). Sheetsof varying thicknesses from 0.043" (1.09 mm) to 0.051" (1.3 mm) werejoined in this manner. In a tensile strength test to determine thestrength of the joint, the joined sheet was placed in a test apparatuswith the joint line transverse to the direction of pull. It was foundthat the MMwPE sheet parted at 23 pounds/0.1875" (10.4 kg/4.76 mm) widthwhile the joint remained intact.

EXAMPLE II

A polyvinyl chloride (PVC) sheet 0.033" (0.84 mm) thick was joined to amillions molecular weight polyethylene (MMwPE) sheet 0.030' (0.76 mm)thick in the manner described above for Example I, excepting only thatthe PVC powder contained 2% of a tin stabilizer. The PVC powder metdirectly with the MMwPE powder with no gradation therebetween. Thepowders were then heated and processed in apparatus similar to that ofFIGS. 1 and 2. In a tensile test of the joint it was found that thejoint parted at 17 lbs/0.1875" (7.7 kg/4.76 mm) width which isapproximately the tensile strength of MMwPE at that thickness.Subsequent tests demonstrated that the joint line strength is increasedby the use of a single mixture of 50% PVC powder and 50% MMwPE powder.Still further improvement in the strength is achieved by use of thegradual increase of one and decrease of the other powder as abovedescribed with reference to the sheets 200, 202 when made respectivelyof polymers A' and B'.

EXAMPLE III

A PVC sheet was joined to a sheet of acrilonitrile butadiene styrene(ABS) while producing both sheets from powders. The sheets were of0.024" (0.61 mm) in thickness. The PVC powder use contained 2% tinstabilizer. A finely powdered ABS resin was spread to meet the edge ofthe PVC powder directly without gradation therebetween. The assembly wasthen subjected to pressure and heat as described above in apparatus likethat shown in FIGS. 1 and 2 to consolidate and form a joint. In astrength test in a tensile apparatus it was found that the ABS sheetparted at 23 lbs./0.1875" (10.4 kg/4.76 mm) width while the jointremained intact. The PVC sheet also remained intact.

EXAMPLE IV

A pre-existing commercially available PVC sheet was joined to a sheet ofMMwPE as the sheet of MMwPE was formed. Pure PVC (without additives)powder was spread to meet the edge of the pre-existing sheet of PVC andMMwPE powder which was to become the MMwPE sheet was spread to meet thePVC powder directly and then processed as in Example I. Tests toseparate the sheet were relatively crude and the tensile strength wasnot measured; however, the sheets did not part at the joint.

Accordingly, the present invention is applicable to the joining of twounlike sheets when both sheets are being formed as in Examples I, II,and III above or where one sheet already exists and the other sheet isformed at the time the joint is made as in Example IV above. In allinstances the surface of the joint reflected the texture of the metalsheet 82 and the parting sheet 84, and was not detectable as a jointexcept as to color when color existed.

From the above examples, it is apparent that virtually any combinationsof plastics may be welded or joined together with weld or jointstrengths close to or equal to that of the sheet itself. In some casesthe strength of the weld or joint is improved by a more gradual changeof one plastic powder material to the other at the joint as previouslydescribed.

It has been noted that when pure PVC or MMwPE are used in this processeither in the formation of sheets or in the formation of welds orjoints, there is comparatively little or no degradation of thesematerials and they are substantially stress free even though thesematerials are not normally considered to be capable of formation intosheets, welds, or joints with less than a great amount of degradationunless the PVC is specially compounded or, in the case of MMwPE, athickness greater than about one-half inch is used. Even with the 392°F. (200° C.) process temperature generally used with these examples, thepresent invention consolidates the plastic material in an extremelyshort dwell period above normal temperatures.

The following table is a useful guide in making first estimates ofwelding or joining temperatures:

    ______________________________________                                                                  Welding  Degrada-                                   Plastic       Melt Point  Temp.    tion                                       ______________________________________                                        Pure Polyvinyl    335° F.                                                                            392° F.                                                                       Very                                     Chloride as       (168° C.)                                                                          (200° C.)                                                                     slight                                   polymerized       (about)                                                     Pure Polyethylene 280-290° F.                                                                        392° F.                                                                       Not                                      or polymerized    (138-143° C.)                                                                      (200° C.)                                                                     measur-                                  millions molecular                   able (too                                weight                               slight)                                  Polyvinyl Chloride                                                                              330-335° F.                                                                        392° F.                                                                       Very                                     plus 2% tin       (166-168° C.)                                                                      (200° C.)                                                                     slight                                   stabilizer                                                                    Pure ABS as       Below 300° F.                                                                      392° F.                                                                       Not                                      polymerized       (149° C.)                                                                          (200° C.)                                                                     measur-                                  (Acrylonitrile-                      able                                     butadiene-styrene)                                                            ______________________________________                                    

The pressures necessary for welding or joining are the same as formaking sheet and are very low and for some plastics may be only thatobtainable from atmosphere and a partial vacuum.

The process and apparatus of this invention are capable of producing anumber of unique new products having improved or new properties as willbe apparent to those skilled in the art. Among these is the ability toproduce wide thickness differences in the same sheet in which case morepowder is used in the thicker areas and less in the thinner areas. Alsodifferent plastics may be used in different portions of the same sheet.Most notably plastic articles including sheets produced by the methodand apparatus of this invention are stress free. Still further, it ispossible by this method and apparatus to produce articles includingsheets of various thicknesses from very thick to very thin films frommaterials which have not previously been considered capable of beingformed into such sheets. Among these are plastic polymers havingmillions molecular weight including, particularly, MMwPE. Mostimportantly, articles including sheets of millions molecular weightplastics such as polyethylene can not only be produced by this inventionwhich was not previously economically possible, but in addition, suchsheets are stress free which also was not previously possible. A productthat is free of stress is very important in a number of applications aswill be appreciated by those skilled in the art since such products aredimensionally very stable under extremes of temperature and do not havethe localized weaknesses often associated with products having stressesbuilt into them during formation. Previous to this invention the use ofplastic sheets was limited to those temperatures that would notexcessively release the stresses built into the product. Sheets or otherarticles produced by this invention, however, can be used at anytemperature the particular plastic will withstand.

While this invention has numerous applications in industry and canproduce a very wide assortment of products including among others, sheetfor use in the construction industry, in cladding, glazing for windows,and the like, one example described below is as a base of substrate fora mirror in place of the usual glass. This particular article isdescribed below with reference to a second and third embodiment of theapparatus of this invention as shown in FIGS. 3 and 4.

Essentially, the apparatus of FIGS. 3 and 4 differs from that of FIGS. 1and 2 in having two diphragms one above and one below the sandwich. Thediaphragms may be either single sheets of a heat resistant flexiblematerial (FIG. 3) or the front face of a sealed envelope (FIG. 4). Thisarrangement permits the use of glass platens which has not heretoforebeen possible.

With reference to FIG. 3, there is shown an apparatus comprising supportmembers S1 and S2 suitably mounted for relative movement. That is to saythat either one or both of the support members S1, S2 may move towardand away from the other support member S1, S2. The upper support memberS1 carries an upper platen 1 while the lower support S2 carries a lowerplaten 2. Each of the platens 1 and 2 incorporates heating cores 3 whichmay be electrical resistance heaters or tubes for carrying heatedfluids, either gas or liquid. In place of heating cores 3 any of theheating mechanisms described above with reference to the apparatus ofFIGS. 1 and 2 may be used.

As shown in FIG. 3, there is a top diaphragm 4 and a lower diaphragm 5carried respectively by pressure frames or members P1 and P2 which mayadvantageously form part of the jaws of a press also incorporating thesupports S1 and S2. The diaphragms 4 and 5 are shown as havingperipheral gaskets 6 of a suitable resiliently compressible sealingmaterial such as rubber. The gaskets are preferably also resistent todeterioration from heat and are preferably spaced a sufficient distancefrom platens 1 and 2 as to reduce the heat that impinges on said gaskets6. The gaskets 6 may be forced into sealing contact by the pressuremembers P1 and P2.

As shown in FIG. 3, a master sheet 7 is positioned between thediaphragms 4 and 5. When producing a mirror, the sheet 7 is preferablyof a high quality tempered or heat resistant glass highly polishedalthough highly polished metal master plates 7 may also be used.

The apparatus also includes a microporous parting sheet 10 and a wirescreen 11 like elements 84 and 85 described above with reference to FIG.1.

In operation, a layer of powdered plastic material 8 is placed upon themaster sheet 7. If desired, a second layer 9 of the same or a differentplastic material and of the same or different color may be placed overthe layer 8. The apparatus is then closed bringing frames P1 and P2 aswell as the support frames S1 and S2 together with the gaskets 6 in gassealing contact to provide a chamber. At the same time, the supportframes S1 and S2 are firmly pressing upon the opposite sides of thesandwich comprised of the master sheet 7, the layer of plastic particles8 (or layers of plastic particles 8, 9, etc.), the microporous partingsheet 10, and the wire screen 11. Either or both of the platens 1, 2 isheated to a temperature sufficient to melt the plastic particles makingup the layer 8 (or the layers 8, 9, etc.). After sealing the gaskets 6and bringing the platens 1 and 2 into firm pressing engagement with thesandwich 7, 8, 9, 10, 11, a partial vacuum is applied through theconduit 12 to the sealed space or chamber defined by the diaphragm 4 and5 and the gaskets 6. In this way, a pressure is achieved on the outerside of the diaphragms 4 and 5 which is a function of the differentialbetween the atmospheric pressure and the residual pressure within thechamber 4, 5, 6 resulting from the applied partial vacuum. This pressurethough relatively low, is sufficient for the formation of many plasticparticles (depending on their molecular weight and their flow propertiesat the melt) into articles and sheets in accordance with the presentinvention. This pressure applies a fluid pressure evenly over the wholearea of the powdered layer 8 (or layers 8, 9, etc.). Also, due to theevenness of the pressure no damage such as cracking, or breaking of theglass master sheet 7 is experienced.

The gaskets 6 are kept sealed and the partial evacuation of the chamber4, 5, 6 maintained long enough for the heat from platens 1 and 2 topenetrate into the plastic layer 8 (or layers 8, 9, etc.) to melt theplastic and to cause it to fuse the applied heat and pressure into acoherent sheet. Thereafter the vacuum at conduit 12 is discontinued, theapparatus is opened, and the finished plastic sheet removed.

FIG. 4 shows an apparatus similar to that of FIG. 3 in which likeelements are numbered with the same reference numerals. In FIG. 4, thepressure members P1, P2 are not utilized nor are the gaskets 6. Also, inFIG. 4, the diaphragms 4 and 5 have been replaced by sealed pressureenvelopes 14 and 15 respectively having flexible diaphragm faces 141,151 respectively. These sealed pressure envelopes 14 and 15 are mountedto support members S1 and S2 respectively by any known suitable means(not shown). It will be appreciated that while the pressure envelopes 14and 15 are shown spaced from the platens 1, 2 for purposes of clarity,the same will be in contact with the platens 1 and 2 and may be secureddirectly thereto as with the envelope 86 shown in FIG. 1. Indeed, theapparatus of FIG. 4 is very similar to that of FIG. 1 accepting that thelower platen 2, unlike the platen 80 in FIG. 1, is also supplied with asealed fluid envelope 15. Conduits 13 and 16 connect the interior of thefluid envelopes 14 and 15 respectively with a source of fluid underpressure. As with the apparatus of FIG. 1 the source of fluid pressuremay be a hydraulic pump, an air compressor, or the like depending uponwhether the fluid is a liquid such as water or hydraulic fluid or a gassuch as air or an inert gas such as nitrogen.

In operation, the support members S1, and S2 are brought together withthe sandwich comprising the master sheet 7, the layer of plasticparticles 8 (or layers 8, 9 etc.), the microporous parting sheet 10, andthe woven wire screen 11 all positioned as shown between the sealedenvelopes 14 and 15. One or both of the platens 1 and 2 will have beenpreviously heated to or above the melting point of the plastic particlesto be molded. After the platens 1 and 2 are exerting a firm pressureupon the sandwich 7, 8, 9, 10, 11 through their respective envelopes 14and 15, the fluid within envelopes 14 and 15 is pressurized by asuitable mechanism through the conduits 13 and 16 respectively. In thisconstruction, it is essential that the platens 1 and 2 be supported bytheir support members S1 and S2 by a mechanism that not only applies thelow pressure required but holds the pressure exerted by the platens 1and 2. The fluid pressure in envelopes 14 and 15 causes the diaphragmfaces 141 and 151 to bear with an even pressure over the entire area ofthe layer of particles to be formed into a sheet. This pressure is veryeven and adjusts automatically to insure evenness over the entire area.The pressure is kept applied in the envelopes 14 and 15 and theapparatus is kept closed to maintain the platens 1 and 2 in position fora time sufficient to achieve melting of the powdered plastic materialand its consolidation into a homogeneous sheet. After formation, thefluid pressure is relieved through conduits 13 and 16 and thus from theenvelopes 14 and 15 and then the apparatus is opened thus moving theplatens 1 and 2 away from each other. The sandwich may then be removedand cooled sufficiently for the plastic to be removed from the masterlayer 7. In many applications it has been found that the fluid pressureapplied to the envelopes 14 and 15 may be from below about 10 p.s.i.(0.73 kg/cm²) to about 50 p.s.i. (3.52 kg/cm²) depending upon suchvariables as the particular plastic being formed, its molecular weightand its flow properties, the nature of any embossings or debossings tobe imparted to the plastic sheet from the master 7 and the like.

An alternative method of operating the apparatus of FIG. 4 comprisesfirst pressurizing the envelopes 14, 15 with fluid pressure while theapparatus is open and support members S1, S2 separates wholly or partly.This pressurizing of the envelopes 14, 15 causes them to balloonoutwardly at a very low pressure such as about 1-5 p.s.i. (0.073-0.365kg/cm²). At this point, the conduits 13, 16 are turned off thusmaintaining the fluid under pressure in the envelopes 14, 15. Thereupon,the apparatus is operated to cause the platens 1, 2 to close togetherwith the necessary additional pressure being supplied by the apparatusitself, for example, with the usual hydraulic or fluid rams such as usedin presses.

In all cases, any air, gas, or moisture which might otherwise tend tobecome trapped within the final sheet (as the plastic powder melts andforms into the sheet) passes out through the microporous parting sheet10, the air escape wire screen 11 and then to the atmosphere with anymoisture being converted to vapor and escaping in the same manner.

The diaphragms 4, 5, 141 and 151 as well as the entire envelopes 14 and15 may be made of any suitable metal or plastic material as mentionedabove that will stand the pressures and temperatures involved. A coppersheet of 0.030" (0.76 mm) in thickness and a stainless steel sheet ofthe 400 series of 0.020" (0.51 mm) in thickness have been usedsuccessfully. Thicknesses of up to about 0.06" (1.5 mm) are also veryuseful.

The coherent plastic sheet produced reflects the high polish of themaster sheet 7 and equals the best that can be produced in glass. Ifdesired the master sheet 7 may be etched or have other surfaceornamentation which will be reproduced in the final plastic sheet.Because of the very even fluid pressure applied by the diaphragms thereis no cracking or breaking of the master sheet 7 even when it is made ofglass.

Because of the extremely smooth and polished surfaces obtainable thesheet produced can be used in many applications where glass is used. Inone application it serves well as a mirror. Indeed reflective materialsbond with greater strength to the plastic sheets produced by thisinvention than to glass. This is contrary to previous experience sinceheretofore polystyrene has been nearly impossible to silver platedirectly without an intermediate coating or other treatment. Also,mirrors made from such sheets provide strikingly better reflectivity.When producing sheets to be used as a mirror care must be taken to avoidany release agent on the surface and if release agents are used on themaster sheet 7 they must be cleaned off thoroughly from the finalplastic sheet. Other than for this, no special treatment such aschemical etchings or corona discharge are necessary.

Silver spray plating has proved very efficient and economical in theproduction of mirrors from sheets produced in accordance with thisinvention. A presently preferred procedure for the application of such areflective coating is as follows:

(a) cleaning and water rinsing to obtain a release free and fingerprintfree surface

(b) a sensitizing spray is applied

(c) a water rinse

(d) a silver solution is sprayed evenly over the surface

(e) a thorough water rinse is applied

All of the above steps (a) through (e) may be performed automatically onequipment conventionally used for glass mirrors. It is presentlypreferred to produce mirrors of the "first surface" type rather than ofthe "second surface" type as is usual with glass. That is to say thatthe reflective coating is placed upon the front surface of the sheet ofplastic rather than the rear surface as is common with glass mirrors. Inorder to protect the silver plating in such first surface mirrors itmust be covered with a suitable transparent abrasion resistant coatingfor example an acrylic coating which is preferably sprayed on.

A preferred material for this abrasion resistant coating is the class ofglass resin polymers described in U.S. Pat. No. 3,451,838. Thesepolymers cure to an abrasion resistant glass-like hard thermosetmaterial that is insoluble in most common solvents non-yellowing andhighly light transmitting above about 1900 A. A 40% solution of such aglass resin in butanol with about 5% of a catalyst has provensuccessful. A thickness of 0.001" (0.025 mm) has been found adequate.

The recommended cure time by the manufacturer of the glass resinutilized was 16-24 hours at 275° F. (135° C.). This recommended curetemperature and time is uneconomically long and the temperatureexcessive for the polystyrene homopolymer mirror sheet 0.09" (2.29 mm)thick which was used to produce a mirror by this invention since thematerial melts at about 280° F. (138° C.). The total of heat and curingtime was reduced to six minutes or less by the use of radiant heatevenly applied from above. This radiation strikes the silver platingwhich reflects back well over 90% of the radiation reaching it. Theresin coating therefore heats very quickly while the other side of thesheet heats only very slowly. A temperature of 400° F. (204° C.) isreached very rapidly (about four minutes or less) and a cure of theglass resin is achieved at this temperature before there is damage tothe underlying polystyrene sheet.

FIG. 5 shows an apparatus for curing in accordance with the justdescribed procedure. As shown in FIG. 5 there is a highly reflectivestainless steel panel 17 having a plurality of radiant heating coils 18positioned therebelow. A polystyrene homopolymer sheet 19 produced bythe present invention either by the apparatus of FIG. 3 or FIG. 4 and0.09" (2.29 mm) thick, having a molecularly thin silver layer 20thereon, and having a film coating 21 of a glass resin as describedabove was cured as shown in the apparatus of FIG. 3. The completeoperation was completed in about six minutes as above mentioned.

Mirrors produced in accordance with this process appear, to the layobserver, to have a greatly magnifying effect due to the fact that sucha mirror does not experience the 10-15% light loss which is common tosecond surface glass mirrors.

In all of the above embodiments of the apparatus and method of thisinvention the heating of the platens may commence before or after theplatens are brought together or before or after fluid pressure isapplied to the plastic particles by the diaphragm or diaphragms. Theparticular sequence will depend upon a number of variables including theparticular equipment being used, time to heat the platens and theplastic, the temperature required, etc. In all instances, however, theplastic particles consolidate into the article while at or above theirmelt temperature and while they are at the same time being subjected tothe fluid pressure applied by the diaphragm or diaphragms.

I claim:
 1. The process of making a plastic article from finely dividedplastic particles comprising:(a) applying a quantity of said plasticparticles to a first member, said particles comprising a first and asecond plastic powder, said first and second powders differing from oneanother, and a layer of the first powder overlying the second powder,whereby the article produced is a laminate; (b) confining a fluid underpressure within a sealed and self-contained flexible envelope; (c)disposing said sealed and self-contained flexible envelope and saidfirst member in opposed relationship with said plastic particlestherebetween; (d) disposing between said plastic particles and saidsealed, self-contained envelope an air release means comprising a porousfabric and a wire cloth with said porous fabric being adjacent saidplastic particles; (e) urging said sealed, self-contained envelopeagainst said air release means and said plastic particles such that saidsealed, self-contained envelope applies a fluid-like pressure on saidplastic particles; (f) applying heat to the plastic particles while theyare under said pressure; (g) venting entrapped gases through said airrelease means from between said particles while the latter are beingheated and pressed with said fluid-like pressure; (h) controlling thedegree of pressure and heat applied to said plastic particles to achievesufficient pressure and heat to cause the particles to melt and to beformed into the shape of said article; (i) releasing said appliedpressure on said plastic; (j) cooling said molten plastic sufficientlyto permit its removal from said first member; and (k) removing saidmolded plastic article from said first member.
 2. The process of makinga plastic article from finely divided plastic particles comprising:(a)applying a quantity of said plastic particles to a first member; saidparticles comprising a first and a second plastic powder, said first andsecond powders differing from one another, and the first and secondpowders being positioned adjacent and in contact with one another; (b)confining a fluid under pressure within a sealed and self-containedflexible envelope; (c) disposing said sealed and self-contained flexibleenvelope and said first member in opposed relationship with said plasticparticles therebetween; (d) disposing between said plastic particles andsaid sealed, self-contained envelope an air release means comprising aporous fabric and a wire cloth with said porous fabric being adjacentsaid plastic particles; (e) urging said sealed, self-contained envelopeagainst said air release means and said plastic particles such that saidsealed, self-contained envelope applies a fluid-like pressure on saidplastic particles; (f) applying heat to the plastic particles while theyare under said pressure; (g) venting entrapped gasses through said airrelease means from between said particles while the latter are beingheated and pressed with said fluid-like pressure; (h) controlling thedegree of pressure and heat applied to said plastic particles to achievesufficient pressure and heat to cause the particles to melt and to beformed into the shape of said article; (i) releasing said appliedpressure on said plastic; (j) cooling said molten plastic sufficientlyto permit its removal from said first member; and (k) removing saidmolded plastic article from said first member.
 3. The process of claim 2wherein said particles are disposed between a first plastic sheet and asecond plastic sheet, whereby a weld is produced joining said first andsecond sheets.
 4. The process of claim 3 wherein said particles comprisea mixture of said first and said second plastic powders, said mixturehaving a composition such that at the midpoint between said sheets itcontains substantially 50 percent of the first powder and 50 percent ofthe second powder.
 5. The process of claim 2 wherein said particlescomprise a mixture of said first plastic powder and said second plasticpowder, disposed between a first plastic sheet and a second plasticsheet, said mixture having a composition such that at its midpoint itcontains substantially 50 percent of the first powder and 50 percent ofthe second powder, the percentage of the first powder in said mixtureincreasing in the direction of the first sheet, and decreasing in thedirection of the second sheet, the percentage of the second powder inthe graded mixture increasing in the direction of the second sheet anddecreasing in the direction of the first sheet.
 6. The process of claim5 wherein the composition of said first plastic sheet is identical tothe composition of said first plastic powder, and the composition ofsaid second plastic sheet is identical to the composition of said secondplastic powder.
 7. The process of making a plastic sheet materialaccording to claims 1 or 2 wherein said flexible envelope is made ofmetal.
 8. The process of making a shaped article, such as sheetmaterial, from finely divided plastic particles comprising:(a) confininga fluid under pressure within a flexible envelope; (b) applying apositive fluid-like pressure with said envelope to plastic particlesarranged in a shaped configuration; (c) venting entrapped gas fromspaces between said particles while said particles are being pressed;(d) continuing the pressing of said particles and heating said particlesto a temperature sufficient to melt the particles and to form saidparticles into the shape of said article under said applied pressure;(e) cooling said molten plastic sufficiently to permit handling, saidparticles comprising a graded mixture of a first plastic powder and asecond plastic powder, said powders differing from one another, saidmixture having a composition such that at the midpoint of the article itcontains substantially 50 percent of the first powder and 50 percent ofthe second powder, the percentage of the first powder in said mixtureincreasing in a first direction away from the midpoint and decreasing inthe opposite direction of said first direction, the percentage of thesecond powder in the graded mixture increasing in said oppositedirection and decreasing in said first direction.